1. Field of Technology
Embodiments disclosed herein relate generally to switching power converters, and more specifically, to techniques for powering a synchronous rectifier controller of a switching power converter.
2. Description of the Related Arts
FIG. 1 is a circuit diagram of a conventional flyback type switching power converter 100 that uses a switch Q1 such as a metal-oxide-semiconductor field-effect transistor (MOSFET). The switching power converter 100 includes a power stage 101 and a secondary output stage 103. Power stage 101 includes the switch Q1 and a power transformer T1. Power transformer T1 includes primary winding Np, secondary winding Ns, and auxiliary winding Na. Secondary output stage 103 includes diode D1 and output capacitor C1. A controller 105 controls the ON state and the OFF state of switch Q1 using output drive signal 107 in the form of a pulse with on-times (TON) and off-times (TOFF).
AC power is received from an AC power source (not shown) and is rectified to provide the unregulated input voltage VDC. The input power is stored in transformer T1 while the switch Q1 is turned on, because the diode D1 becomes reverse biased when the switch Q1 is turned on. The rectified input power is then transferred to an electronic device across the capacitor C1 while the switch Q1 is turned off, because the diode D1 becomes forward biased when the switch Q1 is turned off. Diode D1 functions as an output rectifier and capacitor C1 functions as an output filter. The resulting regulated output voltage VOUT is delivered to the electronic device.
In high output current applications, the conduction loss of the diode D1 operating as the output rectifier is significant. A MOSFET or other actively-controlled switch may replace the diode D1 to minimize conduction loss in the power converter 100 during high output current applications. The MOSFET functions as a synchronous rectifier in the power converter 100. To achieve proper operation of the synchronous rectifier, a synchronous rectifier controller is added to the power converter 100 to control the operation of the synchronous rectifier.
Conventional synchronous rectifier controllers may be powered by the output voltage VOUT of the power converter 100. However, many applications have output voltages lower than the bias voltage required for powering conventional synchronous rectifier controllers. For example, an alternating current (AC)-direct current (DC) charger for mobile devices requires the charger to switch operation between a constant voltage mode where a constant voltage is provided to the mobile devices and a constant current mode where a constant current is provided to the mobile devices depending on the requirements of the mobile devices.
During the constant current mode of the AC-DC charger, the output voltage may drop below the voltage required to power the synchronous rectifier controller. As a result, the synchronous rectifier controller is disabled since the output voltage is insufficient to power the synchronous rectifier controller. When the synchronous rectifier controller is disabled, the body diode of the synchronous rectifier begins to conduct resulting in higher conduction losses and thermal issues. Thus, synchronous rectification is unavailable across the entire operating range of the constant current mode in conventional switched mode power supplies.